It is practically impossible to answer all these passport questions without knowing:
(a) Your nationality
(b) Where you will travel from
Basically, it is almost impossible to travel anywhere these days without photo identity and a passport is a good way of providing this.
There have been questions recently asking if a passport would be necessary to travel from England to Scotland or from Scotland to Northern Ireland. On the face of it, the questions are stupid because these countries are all part of the UK. However, because airlines will require a photo identity document at check-in, having your passport with you would be ideal. The document is not for immigration purposes, but for security reasons.